TRIGGERED FROM INTERNAL STORED FUNCTION.

The problem was that RETURN statement in stored programs did not
clear the Diagnostics Area, as it should have done according to
the SQL Standard.

The user-visible problem was that in some cases an SQL-condition,
raised in the nested function call, would be visible in the outer
scope, which in turn would lead to incorrect handler activation.

The fix is to clear the Diagnostics Area before executing RETURN
statement.

Note, that after the patch, there is no way to throw an SQL-warning
out of a stored function. From the user's view point, he/she will not
get any warning, even if there were warnings, raised during
the function execution.

This change is backward incompatible, but it makes the behaviour of
stored programs more in line with The Standard.